1. Field of the Invention
The present invention relates to an evaporative fuel supply system of an internal combustion engine for introducing evaporative fuel, which is generated within a fuel tank, into an intake passage of the internal combustion engine, and more particularly to an evaporative fuel-purging control system for controlling a purging flow rate of the evaporative fuel.
2. Related Background Art
Conventionally, evaporative fuel-supply systems which introduce evaporative fuel generated in a fuel tank into an intake passage of an internal combustion engine have been widely used for automotive vehicles and the like. In view of preventing air pollution, the evaporative fuel-supply system is used for prevention of evaporative fuel containing a large amount of hydrocarbon (HC) from being emitted from a fuel tank into the atmosphere.
In a typical type of the systems, a canister containing an adsorbent such as activated carbon is used. The evaporative fuel evaporated inside the fuel tank is fed into the canister through a passage connecting the fuel tank with the canister and adsorbed by the adsorbent in the canister. In operation of the internal combustion engine, the adsorbed fuel is removed from the adsorbent in the canister by negative intake pressure generated in the intake passage. The removed evaporative fuel is then purged into the intake passage of the internal combustion engine through a purging passage or purging line connecting the canister with the intake passage.
Purging the evaporative fuel into the intake passage increases the concentration of fuel in gaseous mixture to be supplied to the internal combustion engine, thereby deviating the mixing ratio of the gaseous mixture out of a desired value.
To overcome this disadvantage, Japanese Patent Laid-Open No. 62-174557 (174557/1987) discloses an apparatus which has a solenoid control valve in a purge passage. The solenoid control valve is driven by a pulse waveform signal to control the purging flow rate of evaporative fuel fed into an intake passage. For this apparatus, the pulse waveform signal has a duty ratio and a signal frequency which is substantially inverse-proportional to the duty ratio. During low load such as idling, the duty ratio is decreased in order to lower the purging flow rate. And this makes control signal frequency high, evaporative fuel can be more continuously supplied into the intake passage. Consequently, the mixing condition of the evaporative fuel supplied from the purging passage and the gaseous mixture in the intake passage is somewhat improved.
However, in this apparatus, a high frequency control signal which is supplied to the control valve impairs the controllability of the control valve itself because of the specific response characteristic of the control valve itself. Moreover, the high frequency control signal increases the operating frequency of the control valve and makes the operating noise of the control valve more noticeable.
To overcome this disadvantage, Japanese Utility Model Laid-Open No. 4-1658(1658/1992) discloses an apparatus in which only during idling operation, the frequency of the control signal supplied to the control valve is made lower than the frequency of the control signal in the other operations. The control signal with lowered frequency improves the controllability of the control valve, and the lowered operating frequency of the control valve makes the operating noise of the control value more unnoticeable.
However, in the apparatus as shown in Japanese Utility Model Laid-Open No. 4-1658, during idling operation, because of the low frequency of the control signal to be applied to the control valve, the purging fuel is intermittently supplied to the intake passage. As a result, the mixing condition of the evaporative fuel supplied from the purging passage and the gaseous mixture in the intake passage may be sometimes unsatisfactory. In other words, it is difficult to achieve both the controllability as desired of the purging flow rate at a small duty ratio and the controllability as desired of the purging flow rate at a medium-to-high duty ratio.